A polymer electrolyte fuel cell that uses a proton-conductive polymer electrolyte membrane operates at a low temperature in comparison with other types of fuel cells, for example, such as a solid oxide fuel cell and a molten carbonate fuel cell. Therefore, the polymer electrolyte fuel cell is also expected as a power source for a mobile object such as an automobile, and practical application thereof has also been already started.
In general, a gas diffusion electrode for use in the polymer electrolyte fuel cell is composed of an electrode catalyst layer and a gas diffusion layer. The electrode catalyst layer is a layer that contains catalyst-carrying carbon fine particles coated with the same or different type of ion exchange resin (polyelectrolyte) as or from the polymer electrolyte membrane. Then, the gas diffusion layer has roles of supplying reaction gas to the catalyst layer and collecting electrical charges generated in the catalyst layer. Then, the gas diffusion electrode as described above is joined to the polymer electrolyte membrane in a state where a catalyst layer side thereof is opposed to the polymer electrolyte membrane concerned, whereby a membrane electrode assembly (MEA) is formed. A plurality of the membrane electrode assemblies as described above are stacked on one another while interposing therebetween separators including gas flow passages, whereby the polymer electrolyte fuel cell is configured.
As the gas diffusion layer for use in the polymer electrolyte fuel cell as described above, there is known a layer including a microporous layer, which mainly contains an electrically-conductive substance such as a carbon material, as an intermediate layer on the catalyst layer side of the gas diffusion layer. This intermediate layer is provided in order to lower electrical resistance between the gas diffusion layer and the catalyst layer and to improve a flow of the gas through the gas diffusion layer. As described above, the microporous layer is a layer that composes the gas diffusion layer together with a gas diffusion layer substrate. Hence, in a similar way to the whole of the gas diffusion layer, also for the microporous layer, not only excellent conductivity but also excellent gas permeability is required.
For the microporous layer, in order to allow the microporous layer to ensure drainage properties and strength, a hydrophobic binder is generally used in addition to carbon as such an electrically-conductive agent. As the hydrophobic binder, polytetrafluoroethylene (PTFE) is used in general. For example, in Patent Literature 1, there is disclosed a diffusion layer that contains unbaked and baked PTFE and an electrically-conductive substance.